1. Field of the Invention
The present invention relates to a differential ranging system and method using extremely short pulse width laser beams to measure a distance to or other characteristic of an object.
2. Description of the Related Art
Ultrafast laser oscillators are presently known which are capable of generating pulsewidths on the order of tens of femtoseconds with nanojoule-level pulse energies, at repetition rates as high as 100 Mhz. With this development have come attendant correlation techniques for performing diagnostics on the ultrashort laser pulses. See, e.g., E. P. Ippen and C. V. Shank in Ultrashort Light Pulses, ed. S. L. Shapiro; J. A. Armstrong; J. Appl. Phys., 38 2231 (1967); M. Maier, W. Kaiser and J. A. Giordmaine; Phys. Rev. Lett., 17 1275 (1966); E. P. Ippemn and C. V. Shank; Appl. Phys. Lett., 27 488 (1975); K. L. Sala, G. A. Kenney-Wallace and G. E. Hall; IEEE J. Quantum Electron. 16(9) 990 (1980); J. J. Fontaine, J.-C. Diels, C-Y Wang and H. Sallaba; Opt. Lett., 6(9) 405 (1981)). In these references, it is of course the pulse itself that is the object of scrutiny.
As the field of femtosecond lasers is itself relatively new, the first applications of such lasers are just now being developed. It is one of the objects of the present invention to apply femtosecond laser technology and associated correlation techniques to the field of distance measurement (differential ranging), and related disciplines.
A variety of techniques exist in the prior art for the purpose of ranging between an observer and a remote object, or between two remote objects. Pulsed microwave techniques have provided the means to achieve precision to within a few centimeters over significant distances. Nanosecond pulsed laser radar techniques have provided slightly better distance resolution, down to a few millimeters. Diels et al, in Opt. & Laser Eng., 4 145 (1983), demonstrates the feasibility of using subpicosecond optical pulses to perform ranging with optical time-domain reflectometry. Others have applied this and similar techniques to the problem of imaging objects located within turbid or highly scattering media.